A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
6. What is the purpose of using riprap in drainage channels? (RC1)
Riprap is an erosion-resistant ground cover made up of large, angular and
loose stones (rock, concrete or other material) with geotextile or granular
layer underneath. Riprap is commonly used in drainage channel to provide
a stable lining to resist erosion by channel water. It is also used in channels
where infiltration is intended but the velocity of flow is too large for
vegetation.
A layer of geotextile is normally provided under riprap to perform
separation from underlying soils. This prevents the migration of
fined-grained soils from sub-grade into riprap and results in settlement and
loss of ground.
7. Should riprap be constructed by dumping or by hand-placing?
(RC1)
Riprap by dumping involves the dumping of graded stone by dragline or
crane in such a way that segregation would not take place. Dumped riprap
is a layer of loose stone so that individual stones independently adjust to
shift in or out of the riprap. The dumped riprap is very flexible and would not
be damaged or weakened by minor movement of the bank caused by
settlement. Moreover, local damage or soil loss can be readily repaired by
placement of more rock.
Riprap by hand-placing involves laying of stones by hand and by following
a pattern with the voids between the large stones filled with smaller stones
and the finished surface is kept even. The interlocking riprap produces a
tidy appearance and decreases flow turbulence. Also, owing to the
interlocking nature of riprap it allows the formation of riprap on steeper
bank slopes. The thickness of riprap can usually be reduced when
compared with dumped riprap. However, it requires much labour for
installation of riprap. Another drawback is that the interlocking of individual
rocks produces a less flexible revetment so that a small movement in the
base material of the bank could cause failure of large portions of the
revetment.
8. Should angular or rounded stones be used in riprap channel?
(RC1)
Rock used for riprap should be blocky and angular, with sharp edges and
flat faces. Angular stones proved to be effective to withstand external
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